1. Technical Field
The present invention generally relates to a fuel injection control apparatus, especially for a diesel engine. More specifically, the present invention relates to an improvement to a fuel injection control apparatus that can execute the fuel injection of one cycle in response to the operating conditions by dividing the fuel injection into a main fuel injection and a preceding pilot fuel injection of small quantity.
2. Description of Related Art
In recent years, the fuel injection modes of diesel engines are equipped with a pilot fuel injection mode that executes the fuel injection of one cycle by dividing the fuel injection into a main fuel injection and preceding pilot fuel injection of small quantity. Japanese Laid-Open Patent Publication No. 9-264159 discloses a fuel injection control apparatus for a diesel engine equipped with a pilot fuel injection mode. Under low load conditions, low temperature conditions, and other conditions where the ignitability of the fuel is low, this pilot fuel injection mode improves ignitability and contributes to reducing combustion noise by executing the main fuel injection from a state in which the small quantity of fuel of the pilot fuel injection has been combusted. Meanwhile, in regions where the load and engine speed are somewhat high, the pilot fuel injection is generally not executed and the normal injection mode is used. In short, this type of diesel engine switches between normal injection mode and pilot fuel injection mode in response to the operating conditions.
In view of the above, there exists a need for an improved fuel injection control apparatus. This invention addresses this need in the prior art as well as other needs, which will become apparent to those skilled in the art from this disclosure.
It has been discovered that during pilot fuel injection mode the total fuel injection quantity Q to be fed to the engine is first found in response to the operating conditions. FIG. 15 shows the change in fuel injection quantity and torque when the engine shifts from the pilot fuel injection mode to the normal injection mode. Next, the optimum pilot fuel injection quantity Q1 is determined and the main fuel injection quantity Q2 is found by subtracting the pilot fuel injection quantity Q1 from the total fuel injection quantity Q. Meanwhile, during the normal injection mode, the total fuel injection quantity Q is injected once at a prescribed injection timing.
In short, even during the pilot fuel injection mode, the quantity of fuel fed into the cylinder during one cycle is the same as during the normal injection mode. However, the fuel fed by the pilot fuel injection is injected relatively early and, therefore, generally has an inferior thermal efficiency to that of the fuel supplied during the main fuel injection. That is, even if the quantity of fuel is the same, the pilot fuel injection contributes little to torque generation. During the normal injection mode, in which the fuel is injected with a single injection, the thermal efficiency is relatively high because the fuel is injected at the optimum time. Likewise, the thermal efficiency is relatively high in the main fuel injection because the fuel is injected at the optimum time. Therefore, there is a drawback in that, as shown in FIG. 15, the generated engine torque increases and a torque step occurs when the engine shifts from the pilot fuel injection mode to the normal injection mode, even if the total fuel injection quantity Q is the same.
It is feasible that this kind of torque step could be avoided by taking the value of pilot fuel injection quantity Q1 that is provided in advance by the shape of the map in response to the operating conditions and correcting it further in response to the load. However, it would be difficult to reliably avoid a torque step because the load would change depending on the air-fuel ratio, supercharge pressure, etc., when the engine switched from the pilot fuel injection mode to the normal injection mode.
These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses a preferred embodiment of the present invention.